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1978 911 SC sputtering / bucking in 3rd and 4th gear

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1978 911 SC sputtering / bucking in 3rd and 4th gear

 
Old 01-20-2019, 08:38 PM
  #1  
gforce12
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Default 1978 911 SC sputtering / bucking in 3rd and 4th gear

I have a 78 911 SC, all stock with a recent top end rebuild,Ě when driving at freeway speeds (50+ MPH), car will sputter / buck between 2800 to 3300 RPM in 3rd and 4th gear, the shifter will shake and rattle, this happens when I'm light on the throttle, if I apply a little more gas, car doesn't sputter, car doesn't sputter out of this RPM range or when going faster

My mechanic thought this was due to CIS mixture and has adjusted it several times w/o success, would really appreciate any insight / guidance on how to resolve this issue.

Here's a video of the shifter bucking
https://photos.app.goo.gl/9nobEwGz6xMbhDFW8

Thanks!
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Old 01-21-2019, 03:02 PM
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pmax
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When did this start happening ?

The rpm is solid. Why would your mechanic muck with the CIS ? I would check the engine mounts and ... change the channel.
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Old 01-21-2019, 04:41 PM
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Starbuckslova1
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Not the best person to ask but id start with motor and trans mounts next examine the rods and coupling to the shifter.

Any misfire? Coil wires? Timing ?
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Old 01-21-2019, 05:28 PM
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Dave Inc.
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I'm going to go way out on a limb and say it's some harmonic developing in that rpm range where there's just enough throttle to load up the transmission but all the mounts are so loose that it's vibrating the whole driveline between load and unload. I agree with Starbuckslova, motor/trans mounts and then connections between shifter and trans. That it's happening with 3rd and 4th gear, which are in the neutral position with the trans, seems like an indicator.
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Old 01-21-2019, 06:07 PM
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hot tip , when doing serious troubleshooting , turn off the music so people can hear the engine , couldn't tell anything because of the music,
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Old 01-21-2019, 08:22 PM
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gforce12
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Thank you! I just talked to him today and will let him know to check the mounts, he was speculating it might be a short in the CD unit.

JFYI, he adjusted the mixture and the frequency of the issue def. went down (from 75% to 20% of the time) so he thought there was a correlation.

But if it was the mounts, wouldn't this occur at all RPM range and not just between 2.8 and 3.3?

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Old 01-24-2019, 01:13 AM
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im not really sure but with the mounts you'd feel it when shifting into gear i think.
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Old 01-24-2019, 06:58 PM
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VIdeo link doesn't work. Error message.
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Old 02-09-2019, 06:42 AM
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stlrj
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Has the problem been resolved?
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Old 02-10-2019, 08:28 PM
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gforce12
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Thank you everyone for all the advice, the root cause does appear to be the motor / trans mounts which have now been replaced, I wanted to drive the car for a bit to verify as the problem was intermittent before, I've now driven on the free way ~50 miles in 3rd and 4th in the same RPM range where the shiftier used to buck and it no longer does.



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Old 02-11-2019, 08:13 AM
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It might have been water contamination from your last fillup. It happened to me recently here in Novato with all the recent rains we've been having.
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